1. Field of the Invention
This invention relates in general to atomic absorption spectrophotometry (AAS) and more particularly to methods and apparatus for electrothermally atomizing a sample in preparation for its qualitative or quantitative analysis by AAS.
2. Description of the Prior Art
As is well known, AAS capitalizes on the fact that an element in its atomic state (i.e., in the form of a cloud of atoms) absorbs radiation of a particular wavelength (or frequency) corresponding to the characteristic wavelength which the element emits when appropriately stimulated. This radiation wavelength (or resonance line) is unique for each element; consequently, when a beam of radiation of the wavelength characteristic of the element sought to be determined ("the analyte") is passed through a sample being analyzed in its atomic state, the degree of absorption is proportional to the concentration of the element in the sample which is thus quantitatively analyzed.
The basic apparatus for AAS comprises a source lamp (a hollow cathode or electrodeless discharge lamp) designed to emit radiation of the desired wavelength; means for atomizing the sample to be analyzed; an optical system for forming a beam of the radiation and directing it from the source and through the atomic cloud; and a detector for measuring the intensity of the beam issuing from the atomized sample. This, of course, is a rudimentary system in the nature of a textual schematic drawing. An actual AAS instrument of modern vintage includes various refinements and additional components, e.g., monochromators, background correction means, mechanical and/or electronic beam modulators, etc.
The present invention is concerned primarily with the means by which the sample is atomized. One of these is a burner into which the sample, in solution, is sprayed by a nebulizer; the other, to which the present invention relates, is an electrothermal atomizer that consists of a container (hereinafter referred to as a furnace) configured to allow through passage of the radiation beam and heated to desired temperatures by an electric current passed through the furnace by means of spaced electrodes in contact therewith.
The most common form of furnace, which will be used for exemplary purposes in this description, is a small, thinwalled tube of pyrolytic or pyrolitically-coated graphite, clamped between annular electrodes engaging the ends of the tube and having a sample introduction port in its sidewall at the midpoint of its length. The radiation beam is directed axially through the tube.
While an electrothermal furnace is the atomizer of choice for many elements for which it has lower detection limits and higher sensitivity than flame, it suffers certain disadvantages: one is with respect to the time required for each analysis owing to the fact that after the sample solution is introduced into the furnace, it must be heated to a first, above-ambient "drying temperature", which effects evaporation of the solvent; this accomplished, the heating current is increased to heat the furnace to a higher, "ashing", temperature, which effects decomposition of the residual sample, and, finally, the tube temperature is still further elevated to the (extremely high) temperature required to render the analyte in atomic form. As pointed out by L. de Galan in "Journal of Analytical Atomic Spectrometry, March 1987, vol. 2, pp. 89-93 (at page 89), the typical cycle time for the analysis takes approximately three minutes per sample (including cooling down time) of which only about ten seconds is devoted to the actual analytical step of atomizing the sample, passing the beam through the atom cloud and detecting beam intensity.
Of the total cycle time, approximately one minute each is devoted to drying and ashing, and one minute to cooling and sample introduction. The disproportionate share of time devoted to drying and ashing is necessary because there is a considerable change in volume during the temperature increases and, therefore, rapid, substantially instantaneous heating to atomization temperature would cause the sample to be flushed out of the furnace.
Other shortcomings of conventional electrothermal atomizers are the limited volume of sample liquid which can be accommodated by the graphite tube and the problems inherent in automation of the sample introduction. Typical of currently available "samplers" is that shown in U.S. Pat. No. 4,111,051. It comprises a carousel having at least one ring of sample cups and an arcuately movable sampling tube, the tip of which is alternately positioned in a sample cup where it picks up a measured quantity of sample liquid and in the sample port of the graphite tube where it discharges the liquid. A rinsing station for the pick-up tube is provided to avoid cross-contamination of successive samples.
Various modifications of this basic design have been proposed to improve techniques and apparatus for electrothermal atomization.
Thus, to minimize matrix interference, Sotera, Cristiano, Conley and Kahn, in a technical paper published in Analytical Chemistry, vol. 55, No. 2 (1983) at pages 204-208, using a graphite furnace containing a platform, describe a system in which an aerosol of the sample liquid is sprayed on the platform and/or inner wall of the furnace, using a conventional nebulizer such as commonly used in flame AAS. In the course of the analysis, the furnace can be kept at a temperature higher than ambient (e.g., 400K) enabling a reduction in heating and cooling time required.
A technical paper by Blakeley and Vestal in Analytical Chemistry, vol. 55 (1983), pages 750-754, describes the use of a "thermospray" device for introducing the effluent of a liquid chromatograph (LC) into a mass spectrometer (MS) which functions as a detector. The structure includes a stainless steel tube brazed to a copper block which contains, and is heated by, cartridge heaters. The inlet end of the tube is coupled to the outlet of the LC separating column. The liquid effluent from the column, heated as it flows through the SS tube, emerges as a supersonic stream of vapor, normally containing minute droplets or particles. The stream is directed through an ion source of the mass spectrometer and into a vacuum chamber. The MS takes the form of a quadrupole mass filter which, with an ion lens, causes the customary convergence and divergence of the ion beam.
Another application of a thermospray device enabling utilization of a gas chromatography (GC) detector for LC is described by Yang, Fergusson and Vestal in Analytical Chemistry, vol. 56, (1984), pages 2558-2561. In the described system, the effluent from an LC is deposited by a thermospray device onto a moving belt which conducts it to a standard GC detector.
A more detailed examination of the properties of aerosols by means of a thermospray device and the application thereof to atomic spectroscopy including plasma AES (atomic emission spectroscopy) is reported by Schwartz and Meyer in Spectrochimica Acta, vol. 41 B, No. 12 (1986), pages 1287-1298.
The prior art on the subject includes a paper by Wenrich, Bonitz, Bauer, Niebergal and Dittrich in Talanta, vol. 32, No. 11 (1985), pages 1035-1039, relating to the introduction of an aerosol sample into a graphite tube furnace by means of an ultrasonic atomizer. According to this publication, either the furnace is operated at a constant temperature and sample liquid continuously introduced in the form of an aerosol or the sample can be deposited on the inner wall of the graphite tube and the furnace heated discontinuously. To avoid overloading the furnace and the formation of hydrogen, the sample solvent is vaporized and drawn off in an evaporating unit connected upstream of the furnace.
An electrothermal sample atomization system for AAS providing pre-vaporization of the solvent and having discontinuous heating of the furnace is described by Kantor, Clyburn and Veillon in Analytical Chemistry, vol. 46, No. 14 (1974), pages 2205-2215.